1. Field of the Invention
The present invention relates generally to magnetic sensors in which magnetically sensitive devices are arranged in asymmetric relationship with respect to a magnet and, more particularly, to a gear tooth sensor in which a pair of Hall effect devices are disposed in a common plane which is displaced by a predetermined distance from a central axis of a magnet.
2. Description of the Prior Art
Although magnetically sensitive devices have been used for many years to determine the position of an object, the use of this type of device has increased significantly in recent years because of its use in automotive applications. For example, magnetically sensitive devices are used in both gear tooth sensors and vane switches which are used to control the operation of an internal combustion engine.
U.S. Pat. No. 4,875,008, which issued to Lorenzen on Oct. 17, 1989, describes an angular position sensor for use in association with a rotatable shaft. The sensor utilizes a flat on the shaft and a permanent magnet with its polar axis disposed perpendicularly to the shaft axis. A magneto-resistive element is disposed between the magnet and the flat portion of the shaft. The resistance of the element varies linearly as the shaft is rotated from a first position to a second position.
U.S. Pat. No. 4,853,632, which issued to Nagano et al on Aug. 1, 1989, describes an apparatus for magnetically detecting the position of a moveable magnetic body. The apparatus, which is applicable for sensing the movement of discontinuities in a moveable body, comprises a three terminal magnetic field intensity sensing structure formed by a pair of magneto-resistors. The structure is disposed opposite to a magnetic body which is arranged for movement relative to the structure in a magnetic field. The apparatus generates a first electrical signal of sinusoidal waveform in response to a change in the intensity of the magnetic field due to the relative movement of the magnetic body.
U.S. Pat. No. 4,783,627, which issued to Pagel et al on Nov. 8, 1988, discloses an apparatus for detecting the rotational position of the crank shaft of an internal combustion engine. The apparatus describes a rotor arrangement for use with an automotive Hall generator for detecting the precise position of a select or predetermined one of a plurality of pistons or the precise rotational position of the crank shaft. It provides a distinct marking on a trigger wheel preferable mounted for rotation with the drive shaft of a distributor for generating a signal in a Hall element which is then detected by evaluation circuitry.
U.S. Pat. No. 4,745,363, which issued to Carr et al on May 17, 1988, describes a non-oriented direct coupled gear tooth sensor which uses a Hall cell. It is used to sense gear teeth or similarly shaped discontinuities of a ferrous magnetic or magnetic target. It utilizes a single magnet in association with a direct coupled Hall cell integrated circuit for the purpose of detecting the presence or absence of gear teeth. It incorporates techniques for flux steering and flux reversals independent of the orientation of the sensor to the target to overcome the operate or release point of a Hall switch. The use of spaced apart and offset flux concentrators provides a sensitive device with favorable tolerances in temperature and air gap.
U.S. Pat. No. 4,725,776, which issued to Onodera et al on Feb. 16, 1988, discloses a magnetic position detector which uses a thin film magneto resistor element inclined relative to a moveable object. It employs magneto-resistive elements and detects magnetic teeth of an object to be detected. A constant DC magnetic field is supplied to the magneto-resistive elements in such a way so as to avoid a non-linear region of the DC magnetic field to permit the use of such elements in the region exhibiting good linearity. The present invention provides the DC magnetic field to the magneto-resistive elements by employing a simple structure wherein the magneto-resistive elements are arranged inclined relative to the magnetic field defined between a permanent magnet and the magnetic teeth.
U.S. Pat. No. 4,712,064, which issued to Eckardt et al on Dec. 8, 1987, describes a magneto-resistive sensor for detecting the position or speed of a ferromagnetic body. The sensor is used to emit electric signals in dependency on the position or the speed of a ferromagnetic body. The sensor includes magneto-resistive measuring strips on a substrate which are exposed to a stationary magnetic field such that a large field component extends perpendicular to the measuring strips and a substantially smaller field component in the plane of the measuring strips and at a angle of about 45 degrees to the direction of a measuring current flowing through the measuring strips.
U.S. Pat. No. 4,481,469, which issued to Hauler et al on Nov. 6, 1984, describes a rotary speed sensor with a Hall generator that responds to the tangential component of the magnetic field. In order to determine the rotary speed of a toothed element, such as a starter gear of an internal combustion engine, a magnetic field is generated by a permanent magnet which has both radial and tangential components with respect to the teeth of the rotary element. A magnetic field dependent sensor, such as a Hall generator, is positioned to respond to the tangential component of the magnetic field to thereby eliminate shifts in the null or centerline about which the field strength undulates and the disc rotates to permit ready evaluation of the output from the sensor by threshold circuitry.
U.S. Pat. No. 4,992,731, which issued to Lorenzen on Feb. 12, 1991, discloses a rotary speed sensor that provides base line compensation of the Hall cell output signal. The rotary speed sensor system uses a permanent magnet and a Hall cell sensitive to the changing tangential component of magnetic field caused by the interrupted surface profile of a rotary element. In order to avoid anomalies caused by the variation of the base value of the tangential component of magnetic field, the output of the differential amplifier fed by the Hall cell is connected to a voltage averaging circuit which stores the average voltage of the output on a single compactor. The output of the differential amplifier and the voltage across the capacitor are respectively coupled to the differential inputs of a Schmitt trigger exhibiting hysteresis. The Schmitt trigger thereby establishes operate and release points relative to the average signal. The voltage averaging circuit is a non-linear circuit in which the voltage across the capacitor is fed back to the negative input to an operational amplifier. A power-up circuit is also provided for rapidly charging the capacitor initially to approximately the average value.
It is advantageous for a gear tooth sensing apparatus to be able to distinguish between teeth of different sizes and slots of different sizes so that various coding schemes can be used to determine the accurate position of a rotatable device, such as a crank shaft. In addition, it would be significantly beneficial if the gear tooth sensor is accurately operable on start up without the requirement of rotation of the crank shaft. In other words, a sensor that can determine its position relative to either a tooth or a slot without the need for the rotatable member to rotate is significantly advantageous to an alternative sensor that requires such a rotation prior to its being able to provide the necessary information to control the engine.